This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-087641, filed Mar. 27, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a method of producing formaldehyde, particularly to a novel catalyst that permits producing formaldehyde directly from methane at a high yield and a method of producing formaldehyde by using the novel catalyst.
Formaldehyde is produced by partial oxidation reaction of methanol. Half the methanol produced in an amount of one million tons in a year in Japan is used as a raw material for the production of formaldehyde. The produced formaldehyde is used as a raw material of synthetic resins such as phenolic resins and urea resins or as a raw material of various medicines.
Methanol is synthesized from hydrogen and carbon monoxide obtained by water vapor reforming reaction of methane. The conventional process of producing formaldehyde is as given below:
Methanexe2x86x92H2/COxe2x86x92Methanolxe2x86x92Formaldehyde
The reaction for preparing H2/CO from methane is an endothermic reaction using a large amount of high temperature water vapor, which is one of typical processes consuming a large amount of energy. On the other hand, the reaction for synthesizing methanol from H2/CO is an exothermic reaction. In order to prevent the reaction heat from being generated excessively, the CO conversion rate must be suppressed to about 10% in operating the process unit. Also, the conversion rate of methanol must be suppressed in operating the process unit in the production of formaldehyde by partial oxidation of methanol in order to suppress formation of carbon dioxide and carbon monoxide. In short, the conventional process of producing formaldehyde is a process consuming a large amount of energy and requiring a very complex operation of the process unit.
In order to avoid the large energy-consuming process in the production of formaldehyde, it is necessary to develop a new producing process that permits producing formaldehyde without involving the step of water vapor reformation of methane to produce H2/CO.
It is considered theoretically possible to produce methanol and formaldehyde by partial oxidation of methane, i.e., direct synthesis from methane, as suggested by chemical reaction formulas given below:
CH4+1/2O2xe2x86x92CH3OH, CH4+O2xe2x86x92HCHO+H2O
Therefore, vigorous researches are being made over more than these 50 years on the method of directly synthesizing methanol or formaldehyde from methane in research institutes over the world. Since a catalyst is required for the reactions given above, the major portion of these researches has been concentrated on the development of an effective catalyst. For example, catalysts having molybdenum oxide, vanadium oxide, chromium oxide, etc. supported by silica are reported in, for example, xe2x80x9cChemistry Letter, 1997, p31-32xe2x80x9d and xe2x80x9cCatalyst Today, 45, p29-33(1998)xe2x80x9d.
However, the yield of methanol or formaldehyde is very low, i.e., less than 1% in general, even in the presence of these catalysts. It is said among the researchers in this field that it is difficult for the yield of methanol or formaldehyde to exceed 4%. In other words, the yield of 4% is said to be a wall that cannot be broken in the direct synthesis of methanol or formaldehyde from methane. Further, the methane conversion rate of at least 10% is required for putting the process to practical use. In conclusion, a process for direct conversion from methane into formaldehyde with a practical yield of formaldehyde has not yet been developed.
An object of the present invention is to produce formaldehyde with a high yield directly from methane by a process that does not involve a water vapor reforming step of methane, which is a step consuming a large amount of energy, and that does not bring about an air pollution or water contamination problem. To achieve the object, the present invention provides a novel catalyst and a method of producing formaldehyde by using the novel catalyst.
The present inventors have conducted an extensive research on a catalyst excellent in its activity of partially oxidizing methane and on the conditions of the reaction carried out in the presence of the particular catalyst, and found that a silica-supported 12-molybdosilicic acid catalyst, in which 12-molybdosilicic acid is supported on silica, is excellent in its activity of partially oxidizing methane and, thus, is very effective when used as a catalyst in the synthesis of formaldehyde directly from methane.
12-molybdosilicic acid (which may be hereinafter referred to as SMA in some cases) is poor in thermal stability. Therefore, in using the novel catalyst, it was required to carry out the synthetic reaction of formaldehyde while suppressing the thermal decomposition of the SMA catalyst. The present inventors have continued an extensive research bearing this in mind to arrive at a very important finding. Specifically, it has been found that, for suppressing the thermal decomposition of the SMA catalyst, it is effective to carry out the synthetic reaction of formaldehyde in the presence of the particular catalyst under a water vapor atmosphere. It has also been found that it is very important to control appropriately the temperature elevation rate in heating the reaction system to the reaction temperature.
The present invention, which has been achieved on the basis of the finding given above, is featured as follows:
(1) A method of using a silica-supported 12-molybdosilicic acid catalyst consisting of 12-molybdosilicic acid supported on silica, in which 12-molybdosilicic acid is supported in an amount not smaller than 10 mass % on silica, wherein the reaction system is heated in the presence of the silica-supported 12-molybdosilicic acid catalyst to the operating temperature of the catalyst at a rate not lower than 100xc2x0 C./min.
(2) A method of using a silica-supported 12-molybdosilicic acid catalyst consisting of 12-molybdosilicic acid supported on silica, in which 12-molybdosilicic acid is supported in an amount not smaller than 10 mass % on silica having a specific surface area not smaller than 500 m2/g, wherein the reaction system is heated in the presence of the silica-supported 12-molybdosilicic acid catalyst to the operating temperature of the catalyst at a rate not lower than 100xc2x0 C./min.
(3) A method of producing formaldehyde directly from a mixed gas of methane and oxygen in the presence of a silica-supported 12-molybdosilicic acid catalyst consisting of 12-molybdosilicic acid supported on silica, in which 12-molybdosilicic acid is supported in an amount not smaller than 10 mass % on silica, wherein the reaction system is heated in the presence of the silica-supported 12-molybdosilicic acid catalyst to the operating temperature of the catalyst at a rate not lower than 100xc2x0 C./min.
(4) The method of producing formaldehyde according to item (3), wherein the volume ratio of methane/oxygen in the mixed gas falls within a range of between 9/1 and 4/6, and the reaction temperature in the presence of the catalyst falls within a range of between 550xc2x0 C. and 650xc2x0 C.
(5) The method of producing formaldehyde according to item (3), wherein a water vapor is added to the mixed gas of methane and oxygen in an amount of 40 to 80% by volume based on the total reaction gas consisting of the mixed gas and the water vapor.
(6) The method of producing formaldehyde according to item (4), wherein a water vapor is added to the mixed gas of methane and oxygen in an amount of 40 to 80% by volume based on the total reaction gas consisting of the mixed gas and the water vapor.
According to the present invention, formaldehyde can be synthesized directly from methane with a high yield exceeding 15%, which suggests a breakaway from the conventional process consuming a large amount of energy. It follows that the present invention is expected to make a great contribution to the energy saving in the chemical industries.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.